Structural studies of imidazolium and phosphonium cations with aprotic hetero cyclic anions (AHAs) and investigation of electrochemical CO₂ reduction

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2020-08-10

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Oh, Seungmin

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Abstract

Increasing carbon dioxide (CO₂) concentration in the atmosphere has become a crucial problem because it has greatly contributed to global warming. Previously, Aprotic Heterocyclic Anions (AHAs) based Ionic liquids (ILs) have been developed for post combustion CO₂ capture applications. In order to design the proper combination of ILs which balance the chemical and physical properties, the understanding of fundamental interactions between cations and anions is essential. Particularly, the hydrogen bonding between cations and anions has been identified as a crucial interaction that affects the determination of properties. In this study, we have selected imidazolium and phosphonium cations with good fluidity and thermal stability, pairing them with AHAs with varying basicity and geometry. We investigated the effect of hydrogen bonding in the selected ILs on chemical and physical properties by measuring and correlating the strength of hydrogen bonding with those chemical and physical properties. We found that the role of hydrogen bonding on the determination of physical properties in the selected IL is somewhat different depending on the choice of cation and different geometries of the anions. Furthermore, the structural and chemical knowledge that we gained was applied to the study of electrochemical CO₂ reduction. To focus on the utilization of ILs as additives to effectively catalyze CO₂ reduction on the surface, we have investigated the CO₂ chemistries of the different basic AHAs in diluted organic solvent, acetonitrile (MeCN) with a supporting electrolytes, and eventually compared with the CO₂ chemistries which were observed in the corresponding systems of neat ILs. Then, we have investigated the role of different CO₂ complexes on electrochemical CO₂ reduction. We found that cation-CO₂ complexes with imidazolium ILs did not improve electrochemical CO₂ reduction in non-aqueous systems. Also, unfortunately, anion-CO₂ complexes were not observed in non-aqueous systems due to the solvent effect. Finally, upon addition of water, the formation of bicarbonate with the reprotonated anions led to a slight improvement in electrochemical CO₂ reduction

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